In the field of self guided vehicles (SGV), accurately sensing the vehicle velocity and position is of particular importance. Obviously, the vehicle must "know" its velocity to properly guide itself along a desired path. Moreover, the basic velocity information is also used to determine instantaneous vehicle position and is important in precisely locating the vehicle. Accordingly, it is not uncommon to employ a high accuracy resolver along with a corresponding resolver to digital converter (R/D) to detect angular wheel position and vehicle velocity. To protect the vehicle from a loss of the resolver signals, copending U.S. application 62,037 filed June 15, 1987 now U.S. Pat. No. 4,740,778 by Harding et al illustrates a circuit for monitoring the resolver output and shutting down the vehicle in response to a loss of the carrier signal.
Additionally, faulty operation can occur due to a failure of the R/D. Failure of the R/D has at least three distinct modes. The R/D may fail with an output that is erratic, an output that indicates the vehicle is moving, or an output that indicates the vehicle is stationary. While any of the three failures is undesirable, the most critical takes place when the vehicle is actually moving and the failed R/D indicates a stationary vehicle. During this type of failure, the vehicle can inadvertently move without guidance assistance. The other two types of faults will lead to calculated positions which are highly inaccurate, but the vehicle will only move under direction from the guidance controller. It is desirable to provide a single, reliable apparatus for detecting any failure of the R/D.
The present invention is directed to overcoming one or more of the problems as set forth above.